The present invention relates to a super absorbent polymer composition in which a super absorbent polymer has improved stability. More particularly, it relates to a super absorbent polymer composition in which a super absorbent polymer is protected against decomposition and/or deterioration even in a water-containing state, i.e., containing an aqueous liquid or a body fluid such as urine, blood and sweat.
Super absorbent polymers have been used widely as an absorbent material in disposable diapers for babies, adults and persons suffering from incontinence or sanitary napkins in the field of sanitation; water-retaining materials in the field of agriculture and horticulture; and coagulants for sludge, materials for preventing dew drop condensation or water stopping materials in the field of construction. It is known that water-soluble high-molecular weight compounds constituting such super absorbent polymers undergo reduction in molecular weight and deterioration with time in the presence of radical generating species, such as hydrogen peroxide or L-ascorbic acid or salts thereof.
Body fluids such as urine, blood and sweat contain L-ascorbic acid or salts thereof. Therefore, it has been a great problem that a super absorbent polymer used in a disposable diaper or a sanitary napkin having absorbed therein such a body fluid undergoes decomposition and deterioration with time by the action of the radical species generated from L-ascorbic acid or salts thereof and therefore reduces its capacity of retaining the body fluid.
The decomposition of a super absorbent polymer due to radical generating species is pronounced in its water-containing state in an air atmosphere, especially in the presence of an aqueous solution containing transition metal ions, such as iron and copper, capable of having an oxidation number of 2 or more.
This is because a trace amount of a transition metal ion, such as iron and copper, acts as a catalyst to markedly accelerate the decomposition reaction (radical generating reaction) of hydrogen peroxide, or L-ascorbic acid or salts thereof, as described in J. Am. Chem. Soc., 89, No. 16, 4176 (1967) and Free Radical Research Communications, 1, No. 6, 349 (1986).
These transition metal ions are known, like radical generating species such as hydrogen peroxide or L-ascorbic acid or salts thereof, to cause depolymerization of water-soluble polymers with time (see, e.g., Carbohydrate Research, 4, 63 (1967)).
Means that have been widely employed for suppressing decomposition and deterioration of super absorbent polymers include (1) sealing of a super absorbent polymer under reduced pressure or in a nitrogen atmosphere to avoid contact with air (especially with oxygen), (2) use of highly purified water and reagents to inhibit incorporation of metal ions into a super absorbent polymer, (3) addition of an antioxidant or a reducing agent to a super absorbent polymer, (4) addition of a protein or an enzyme to a super absorbent polymer, and (5) addition of a metal chelating agent, such as citric acid, (poly)phosphoric acid or salts thereof, or ethylenediaminetetraacetic acid (EDTA) or salts thereof, to a super absorbent polymer (JP-A-63-146964 corresponding to EP-A-257951).
In many cases, however, means (1) and (2) are impossible to carry out for some end uses of a super absorbent polymer. Means (3), (4) and (5) including addition of a known additive suppress decomposition and deterioration of a super absorbent polymer, but the effect exerted is not always sufficient. Cases are often met with in which the additive must be added in a large quantity or an additive having a very strong action must be used. Under such a situation, it is likely that the inherent physical properties or performances of super absorbent polymers should be impaired seriously. Additionally, where the system containing a super absorbent polymer is used in direct contact with a human body or a plant or an animal, adverse influences on the living system give rise to a problem.
In fact, various methods for suppressing decomposition and deterioration of super absorbent polymers have been proposed to date. For example, Japanese Patent Application Laid-Open 63-272349 (corresponding to EP-A-268459, U.S. Pat. No. 4,959,060), Japanese Patent Publication 5-34383 (corresponding to EP-A-249391, U.S. Pat. No. 4,972,019), and Japanese Patent Application Laid-Open 2-255804 (corresponding to EP-A-372981) and 3-179008 disclose a method of using additives such as a sulfur-containing reducing agent, an oxygen-containing reducing inorganic salt, a water-soluble chain transfer agent. Nevertheless, these additives have a disadvantage of giving off an offensive odor or being an irritant and/or an allergen and need sufficient care for safety when actually used under such conditions that the additive may contact with a human body. Thus, an additive having higher safety has been keenly demanded.
Accordingly, an object of the present invention is to provide a super absorbent polymer composition in which the super absorbent polymer can exist stably without undergoing decomposition and deterioration even in the presence of an aqueous solution or water containing radical generating species, such as hydrogen peroxide or L-ascorbic acid or salts thereof, and which has high safety for a living system.
The present inventors have conducted extensive investigations to solve the above-mentioned problem. As a result, they have found that a super absorbent polymer in a water-absorbing state can be stabilized by inhibiting a decomposition reaction (radical generating reaction) of radical generating species, such as hydrogen peroxide or L-ascorbic acid or salts thereof and that this can be achieved by completely sequestering a transition metal ion acting as a catalyst for the radical generating reaction by converting the transition metal ion into a water-insoluble chelate.
The present invention has been reached based on the above finding. That is, the above object of the present invention is accomplished by a super absorbent polymer composition comprising a chelating compound (A) which has a site capable of forming a chelate with a copper ion and whose copper salt has a solubility in physiological saline at 25xc2x0 C. of 0.01% by weight or less (excluding xcex2-diketone derivatives) and a super absorbent polymer (B), wherein said chelating compound (A) is present in an amount of from 0.0001 to 30 parts by weight per 100 parts by weight of said super absorbent polymer (B).
The super absorbent polymer composition of the present invention is effectively used, though dependent on the kind of the super absorbent polymer used, in combination with cosmetics or food additives containing radical generating species, such as ascorbic acid. The super absorbent polymer composition of the present invention is particularly suitable as an absorbent material of sanitary articles.
The super absorbent polymer composition of the present invention will be described below in detail.
The super absorbent polymer (B) which can be used in the present invention is not particularly limited. Examples of usable super absorbent polymer (B) include partially crosslinked polymers having a carboxyl group or salts thereof, such as a crosslinked polyacrylic acid salt, a crosslinked poly(vinyl alcohol/acrylic acid salt) copolymer, a (crosslinked) starch-acrylic acid salt graft copolymer, and a crosslinked polyvinyl alcohol-poly(maleic anhydride salt) graft copolymer; and partially crosslinked polysaccharides, such as a crosslinked carboxymethyl cellulose salt. From the viewpoint of water absorption capacity, it is preferable to use a crosslinked polyacrylic acid salt or a (crosslinked) starch-acrylic acid salt graft copolymer, with a crosslinked polyacrylic acid salt being especially preferred.
These super absorbent polymers (B) may be used either individually or in combination of two or more thereof.
The salt in the super absorbent polymers includes, for example, an alkali metal salt, an alkaline earth metal salt, and an ammonium salt. The degree of neutralization of the super absorbent polymers is from 0.01 to 100%, preferably from 1 to 99%, still preferably from 40 to 95%, based on the number of moles of the acid groups in the super absorbent polymer.
The chelating compound (A) which can be used in the present invention has a site capable of forming a chelate with a copper ion, and the copper salt thereof has a solubility in physiological saline at 25xc2x0 C. of 0.01% by weight or less. The term xe2x80x9csite capable of forming a chelate with a copper ionxe2x80x9d as used herein means a site capable of forming a coordination bond with a metal ion in a compound which is generally considered to act as a ligand in the field of complex chemistry. The term xe2x80x9csolubilityxe2x80x9d as used herein means a concentration (wt %) obtained by dividing [a maximum amount (weight) of a copper salt of the chelating agent which dissolves in physiological saline at 25xc2x0 C. to make a clear solution after stirred for 30 minutes] by [the weight of the solution].
The chelating compound (A) is safer than conventional additives that have been used as a stabilizer for super absorbent polymers and is yet equal or superior to those additives in stabilizing effect for super absorbent polymers.
The chelating compound (A) is preferably used in an amount of from 0.0001 to 30 parts by weight, still preferably from 0.001 to 10 parts by weight, and particularly from 0.01 to 5 parts by weight, per 100 parts by weight of the super absorbent polymer. Addition of less than 0.0001 part of chelating compound (A) produces no effect. Addition of more than 30 parts by weight shows no further improvement, rather tending to impair the physical properties of the super absorbent polymer. Therefore, the amount of chelating compound (A) to be used preferably falls within the above-specified range.
It is particularly preferable that chelating compound (A) is capable of forming such a chelate with Cu2+ ion at 25xc2x0 C., expressed in a common logarithm value (hereinafter sometimes referred to as pKCu), is about 3 or greater. If the pKCu is less than about 3, the chelating compound sometimes has insufficient performance as a stabilizer for a super absorbent polymer.
No limitation is imposed on the type of the chelating compound (A) to be used in the present invention as far as the above-mentioned physical properties are satisfied. Preferred examples of the chelating compounds (A) include the following compounds (1) and (2):
(1) Compounds which comprise a hydrophobic moiety having a saturated or an unsaturated hydrocarbon group having 6 or more carbon atoms and a hydrophilic moiety having at least one group selected from the group consisting of a carboxyl group, a sulfo group, a hydroxyl group and a phospho group; and
(2) Tropolone derivatives.
In the present invention, one or more compounds selected from any of the compounds (1) and (2) can be used. Compounds selected respectively from at least 2 groups (1) and (2) may be used in combination.
It is also preferable that the chelating compounds in combination have a pKCu of about 3 or higher.
The chelating compound (A) belonging to the group (1) comprises a hydrophobic moiety having a saturated or an unsaturated hydrocarbon group having 6 or more carbon atoms (preferably 6 to 30, more preferably 12 to 22) and a hydrophilic moiety having at least one group selected from the group consisting of a carboxyl group, a sulfo group, a hydroxyl group and a phospho group. The saturated hydrocarbon group includes a straight-chain or branched alkyl group, or a cycloalkyl group. The unsaturated hydrocarbon group includes a straight-chain or branched alkenyl group, or a phenyl group. Examples of chelating compounds (A) having such groups are polycarboxylic acid derivatives, hydroxycarboxylic acid derivatives, iminodiacetic acid derivatives, organic acid amide derivatives, N-acyl amino acid derivatives, phosphate ester derivatives phosphonic acid derivatives and polyphosphonic acid derivatives as well as their alkali metal salts and alkaline earth metal salts.
The polycarboxylic acid derivatives include alkylmalonic acids and alkenylmalonic acids as well as salts thereof. The hydroxycarboxylic acid derivatives include monoalkyl citrates and monoalkenyl citrates as well as salts thereof. The iminodiacetic acid derivatives include N-alkyl-Nxe2x80x2-carboxymethylaspartic acids, N-alkenyl-Nxe2x80x2-carboxymethylaspartic acids and salts thereof. The organic acid amide derivatives include citric acid monoalkylamides, citric acid monoalkenylamides and salts thereof. The N-acyl amino acid derivatives include an N-acyl glutamic acid, an N-acyl aspartic acid and salts thereof. The phosphate ester derivatives include monoalkyl phosphates, monoalkenyl phosphates and salts thereof. The phosphonic acid derivatives include alkylphosphonic acid, alkenylphosphonic acid and salts thereof, and phenylphosphonic acid and salts thereof. The polyphosphonic acid derivatives include alkylenebis(nitrilodimethylene)tetraphosphonic acids and salts thereof.
Preferred among them are citric acid monoalkylamides, citric acid monoalkenylamides and salts thereof; monoalkyl citrates, monoalkenyl citrates and salts thereof; alkylmalonic acids, alkenylmalonic acids and salts thereof; N-alkyl-Nxe2x80x2-carboxymethylaspartic acids, N-alkenyl-Nxe2x80x2-carboxymethylaspartic acids and salts thereof; N-acyl glutamic acids and salts thereof; and monoalkyl phosphates, monoalkenyl phosphates and salts thereof. In particular, citric acid derivatives including citric acid monoalkylamides, citric acid monoalkenylamides and salts thereof, and monoalkyl citrates, monoalkenyl citrates and salts thereof are still preferred for their high effects on stabilizing super absorbent polymers. N-acyl amino acid derivatives are also preferred for the same reason.
The above-mentioned citric acid monoalkylamides, citric acid monoalkenylamides and salts thereof preferably include those represented by formula (I): 
wherein R1 represents an alkyl or alkenyl group having 6 to 30 carbon atoms; and M1""s, which may be the same or different, each represents an alkali metal ion, an ammonium ion or a hydrogen.
The citric acid monoalkylamides, citric acid monoalkenylamides and salts thereof can be synthesized by known methods. For example, they are obtained by subjecting an amine and citric acid to complete dehydrating condensation to form an imine, which is then hydrolyzed, followed by neutralization. A proper choice of the number of carbon atoms of R1 in formula (I) affords a citric acid monoalkylamide, a citric acid monoalkenylamide or salts thereof that meets the purpose. If the number of carbon atoms of R1 exceeds 30, the resulting compound has considerably reduced water solubility. If it is less than 6, the performance of the resulting compound as a stabilizer for super absorbent polymers is reduced. The number of carbon atoms of R1 preferably ranges from 12 to 22.
The monoalkyl citrates, monoalkenyl citrates and salts thereof preferably include those represented by formula (II): 
wherein R2 represents an alkyl or alkenyl group having 6 to 30 carbon atoms; and M2""s, which may be the same or different, each represents an alkali metal ion, an ammonium ion or a hydrogen.
These monoalkyl citrates, monoalkenyl citrates and salts thereof can be synthesized by known methods. For example, they are obtained through dehydrating condensation of an alcohol and citric acid. A proper choice of the number of carbon atoms of R2 in formula (II) affords a monoalkyl citrate, a monoalkenyl citrate or salts thereof that meets the purpose. If the number of carbon atoms of R2 exceeds 30, the resulting compound has considerably reduced water solubility. If it is less than 6, the performance of the resulting compound as a stabilizer for super absorbent polymers is reduced. The number of carbon atoms of R2 preferably ranges from 12 to 22.
The alkylmalonic acids, alkenylmalonic acids and salts thereof preferably include those represented by formula (III): 
wherein R3 represents an alkyl or alkenyl group having 6 to 30 carbon atoms; and M3""s, which may be the same or different, each represents an alkali metal ion, an ammonium ion or a hydrogen.
These alkylmalonic acids, alkenylmalonic acids and salts thereof can be synthesized by known methods. For example, they are obtained by adding an xcex1-olef in to methyl malonate or ethyl malonate to obtain a methyl alkylmalonate or an ethyl alkylmalonate, which is then hydrolyzed, followed by neutralization. A proper choice of the number of carbon atoms of R3 in formula (III) affords an alkylmalonic acid, an alkenylmalonic acid or salts thereof that meets the purpose. If the number of carbon atoms of R3 exceeds 30, the resulting compound has considerably reduced water solubility. If it is less than 6, the performance of the resulting compound as a stabilizer for super absorbent polymers is reduced. The number of carbon atoms of R3 preferably ranges from 12 to 22.
The above-mentioned N-alkyl-Nxe2x80x2-carboxymethylaspartic acid, N-alkenyl-Nxe2x80x2-carboxymethylaspartic acid and salts thereof preferably include those represented by formula (IV): 
wherein R4 represents an alkyl or alkenyl group having 6 to 30 carbon atoms; and M4""s, which may be the same or different, each represents an alkali metal ion, an ammonium ion or a hydrogen.
These N-alkyl-Nxe2x80x2-carboxymethylaspartic acids, N-alkenyl-Nxe2x80x2-carboxymethylaspartic acids and salts thereof can be synthesized by known methods. For example, they are obtained by adding an amine to maleic acid to obtain an alkylaminosuccinic acid, which is then carboxymethylated with carboxymethyl chloride, followed by neutralization. A proper choice of the number of carbon atoms of R4 in formula (IV) affords an N-alkyl-Nxe2x80x2-carboxymethylaspartic acid, an N-alkenyl-Nxe2x80x2-carboxymethylaspartic acid or salts thereof that meets the purpose. If the number of carbon atoms of R4 exceeds 30, the resulting compound has considerably reduced water solubility. If it is less than 6, the performance of the resulting compound as a stabilizer for super absorbent polymers is reduced. The number of carbon atoms of R4 preferably ranges from 12 to 22.
The monoalkyl phosphates, monoalkenyl phosphates and salts thereof preferably include those represented by formula (V): 
wherein R5 represents an alkyl or alkenyl group having 6 to 30 carbon atoms; and M5""s, which may be the same or different, each represents an alkali metal ion, an ammonium ion or a hydrogen.
The above monoalkyl phosphates, monoalkenyl phosphates and salts thereof can be synthesized by known methods. For example, they are obtained by esterifying an alcohol with phosphorus pentoxide, phosphorus oxychloride or polyphosphoric acid. A proper choice of the number of carbon atoms of R5 in formula (V) affords a monoalkyl phosphate, a monoalkenyl phosphate or salts thereof that meets the purpose. If the number of carbon atoms of R5 exceeds 30, the resulting compound has considerably reduced water solubility. If it is less than 6, the performance of the resulting compound as a stabilizer for super absorbent polymers is reduced. The number of carbon atoms of R5 preferably ranges from 12 to 22.
The above-mentioned N-acyl glutamic acid and the salts thereof are preferably represented by the following formula (VI). 
wherein R6xe2x80x94COxe2x80x94 represents acyl group having 6 to 30 carbon atoms, M6""s which may be the same or different, each represents alkali metal ion, an ammonium ion, a triethanol ammonium ion or a hydrogen.
The N-acyl glutamic acid and salts thereof can be synthesized by known method, and it is also commercially available. A proper choice of the number of carbon atoms of R6xe2x80x94COxe2x80x94 in formulation (VI) affords N-acyl glutamic acid or salts thereof which meets the purpose. If the number of the carbon atoms in R6xe2x80x94COxe2x80x94 is less than 6, performance of the super absorbent resin as a stabilizer is reduced. The number of carbon atoms of R6xe2x80x94COxe2x80x94 preferably ranges from 12 to 22.
The N-acyl aspartic acid and salts thereof preferably include those represented by formula (VII). 
wherein R7xe2x80x94COxe2x80x94 represents acyl group having 6 to 30 carbon atoms, M7""s which may be the same or different, each represents an alkali metal ion, an ammonium ion, a triethanol ammonium ion or a hydrogen.
The N-acyl aspartic acid and salts thereof can be synthesized by known method, and it is also commercially available. A proper choice of the number of carbon atoms of R7xe2x80x94COxe2x80x94 in formula (VII) affords a N-acyl aspartic acid or salts thereof which meets the purpose. If the number of the carbon atoms of R7xe2x80x94COxe2x80x94 exceeds 30, the resulting compound has considerably reduced water solubility. If it is less than 6, the performance of the resulting compound as a stabilizer for super absorbent polymers is reduced. The number of the carbon atoms of R7xe2x80x94COxe2x80x94 preferably ranges from 12 to 22.
The chelating compounds (A) of the above-mentioned compounds (1) are safer and more effective to inhibit decomposition and deterioration of super absorbent polymers than the additives conventionally employed as a stabilizer for super absorbent polymers.
The chelating compounds (A) of the compounds (1) may be used either individually or in combination of two or more thereof.
The tropolone derivatives of the above-mentioned compounds (2) are compounds capable of forming a chelate with a copper ion, and the copper salts of the tropolone derivatives have a solubility in physiological saline at 25xc2x0 C. of 0.01% by weight or less. Such tropolone derivatives are naturally-occurring substances found in certain kinds of trees. They have high safety having no serious influences on a human body, an animal or a plant even in contact in a high concentration. Use of the tropolone derivative for stabilization of super absorbent polymers, i.e., prevention of decomposition and deterioration of super absorbent polymers has been unknown. In particular, use of a super absorbent polymer composition containing the tropolone derivative as an absorbent material of sanitary articles, such as absorbing articles, has been unknown. They are safer and more effective to inhibit decomposition and deterioration of super absorbent polymers than the additives conventionally employed as a stabilizer for super absorbent polymers.
While specific but not-limiting examples of the tropolone derivatives are tropolone, xcex2-thujaplicin, xcex3-thujaplicin, xcex2-dolabrin and methyl 6-isopropyltropolone-4-carboxylate, as well as sodium or potassium salt thereof. Preferred of them are xcex2-thujaplicin and xcex3-thujaplicin. Inter alia, xcex2-thujaplicin is preferred for its safety to a living body as having been used as a perfume for hair tonics, toothpaste, perfume preparations, external preparations, bath agents, shampoos and rinses.
The tropolone derivatives may be synthetics or semi-synthetics. It is also possible to use natural hiba oil or hinoki oil containing these tropolone derivatives either as such or after purification.
These tropolone derivatives may be used either individually or in combination of two or more thereof.
The super absorbent polymer composition according to the present invention may contain water in addition to the components (A) and (B) just like a water-containing polymer or a water-containing gel. The composition may be formulated into an aqueous composition by dispersing the super absorbent polymer in water or mixing the polymer with water like an aqueous dispersion or emulsion. If desired, the composition of the present invention may contain various additives, such as a water-soluble organic solvent, a surface active agent, salts, a stabilizer, an antioxidant and/or an antiseptic.
While the super absorbent polymer of the present invention may have any form as far as it contains the components (A) and (B), it is preferably used in the form of an aqueous dispersion or emulsion containing components (A) and (B), a solid mixture of components (A) and (B), and a solid of component (B) impregnated with component (A). A form in which component (B) contains therein component (A) is particularly preferred.
The super absorbent polymer composition of the present invention can be prepared, for example, by the following methods:
(1) A method in which solid components (A) and solid component (B) are ground and mixed;
(2) A method in which component (B) is impregnated with an aqueous solution or dispersion of component (A), followed by drying;
(3) A method in which component (B) is impregnated with a solution of component (A) in a hydrophilic organic solvent, followed by drying;
(4) A method in which an aqueous solution or organic solvent solution of component (A) is applied to component (B), followed by drying;
(5) A method in which component (A) is heat-melted, applied to component (B), and cooled; and
(6) A method in which components (A) and (B) are mixed in water and used as such.
As stated previously, the super absorbent polymer composition of the present invention is particularly useful as a water absorbing material in sanitary articles, such as absorbent articles, e.g., disposable diapers and sanitary napkins.
These absorbent articles comprise a water-permeable topsheet, a water-impermeable back sheet and an absorbent member interposed between the topsheet and the back sheet. The absorbent member to be used comprises fluff pulp, i.e., comminuted wood pulp, in combination with the super absorbent polymer composition of the present invention. The super absorbent polymer composition may be mixed with fluff pulp or be provided in the form of a layer in a specific part of the absorbent member, i.e., in any of the upper layer, the middle layer and the lower layer. In another embodiment, a mixture of a thermoplastic resin, fluff pulp and the super absorbent polymer composition of the present invention may be heat-treated to provide an integral absorbent member. Since a body fluid such as urine contains substances such as L-ascorbic acid or a salts thereof as previously mentioned, a super absorbent polymer in an absorbent article having absorbed therein a body fluid undergoes deterioration caused by these substances. According to the present invention, such deterioration of a super absorbent polymer can be suppressed by using the super absorbent polymer composition of the present invention as an absorbent material of an absorbent article. Absorbent articles using the super absorbent polymer composition of the present invention hardly cause a back-flow of the absorbed body fluid which is ascribed to deterioration of the super absorbent polymer. As a result, absorbent articles such as disposal diapers and sanitary napkins can be used for a long time, for example, in the nighttime with freedom from care.